An electrical or communications bus (or simply bus) is a physical electrical interface where more than one device shares the same electric connection. Two-wire electrical buses are particularly suitable for use in vehicle systems as they are more robust and simple to install and configure. Various communication protocols may be used on such electrical buses and used to provide communications between electronic components located throughout a vehicle. In particular, airbag systems have many types of components that may be connected to a network. Airbag systems require robust and failsafe communications between the various components to avoid critical system failures or incorrect airbag deployment.
“DSI bus standard version 2.02”, 29 Mar. 2005, TRW Automotive and Freescale Semiconductor describes the Distributed System Interface (DSI) message protocol. The DSI is a single master device multiple slave device data communications (electrical) bus implemented on two wires. The bus utilises voltage modulation (pulse-width modulation) signalling for digital messages sent from the single master device (master) to the slave devices (slaves) and current mode signalling from the slave devices to the master device. Slave devices only transmit in response to messages sent from the master device and are synchronised to the master signals.
A bit time is the duration of one bit or the reciprocal of the transmission frequency. For example, a 1 kHz (1 kbps) signal has a bit time of 1 ms. In the DSI message protocol the master voltage modulation is formed by separating each bit time into thirds. For a master logical zero the master produces a signal that is low for ⅔ of the bit time and high for the final ⅓ (a top hat voltage pulse). For a master logical one the master device produces a voltage signal that is low for ⅓ of the bit time and high for the remaining ⅔ of the bit time, i.e. the last ⅔ portion of the bit time.
FIG. 2 shows a schematic diagram illustrating logical one and logical zero master logic signals according to the DSI message protocol.
According to the DSI message protocol slave responses to commands are sent using a modulated current signal, which is self synchronised to a falling edge voltage from the master voltage signal. Master and slave devices transmit simultaneously, i.e. the DSI is a duplex system of communication. During the response time the master device sends a pulse train of any combination of ones or zeros. The current mode bits are sent during the bit time and sampled by the master device at the falling edge of the voltage pulse generated by the master device. When the slave device produces a logical one, the slave device draws additional current above a particular threshold from the electrical bus during the bit time. If a logical zero is transmitted the slave device does not draw additional current during the bit time. The master device detects the current drawn from the electrical bus at a predetermined point in the bit time, i.e. the sampling point, to determine the bit type transmitted by the slave device.
FIG. 3 shows a schematic diagram of the slave logical one and slave logical zero current signals, respectively. The dotted lines in FIGS. 3 and 4 indicate the sampling point, which in the DSI message protocol is on the falling voltage edge of the master signal, i.e. at the end of the bit time. FIG. 4 shows the decoded slave logical signal for any point in the bit time, i.e. how the master receiver would decode the current at any particular point in time.